Cyclophosphamide inhibited the immune response that follows intravitreal injection of bovine gamma globulin (BGG) into the rabbit vitreous. There were no antibody producing cells in the uveal tracts, and no antibody in serum, aqueous humor or vitreous humor. The drug was effective when treatment began as late as five days after immunization and continued until day 13. Cyclophosphamide suppressed the secondary response to intravitreally injected BGG if treatment was begun before the challenge injections. It was not effective when given after challenge. Cyclophosphamide did not inhibit priming for a secondary response. Lymph nodes of intravitreally immunized rabbits produced migration inhibition factor (MIF). In some cases uveal tract cell also produced MIF. Guinea pigs immunized intravitreally with BGG did not show a state of systematic delayed hypersensitivity, and no antibody was detected in their serum or ocular field.